Question

The charge for telephone calls T, is partly constant and partly varies with the number n of unit of call. The bill for 580 units of call is 900 .while the bill for 310 units is 500.

Answer 1

think of it as being given 2 ordered pairs (580,900) and (310,500)
of the type (x,y) where x is number of units, and y is cost
slope = (900-500)/(580-310) = 400/270 = 40/27

y = mx + b is you slope y-intercept form
using one of the points, and the slope ...
900 = (40/27)(580) = b
b = 1100/27

y = (40/27)x + 1100/27
or
cost = (40/27)(number of items) + 1100/27

testing with the point not used, (310,500)
LS = 500
RS = (40/27)(310) + 1100/27 = 500
= LS

My answer is correct.